How to smooth out different interpolation types
An easier way to roll a ball downhill is to use rigid bodies. This way, the active objects are subject to the scene gravity.

For this, you will want two rigid bodies, one active and one passive. The active one will be the ball, the passive will be slope.
Add rigid body physics to each object in the physics tab:

These are the settings for the ball:

These are the settings for our slope:

A note on the difference between active and passive rigid bodies from the manual

Active
The object is dynamic and is directly controlled by simulation results.
Passive
The object remains static and is directly controlled by animation system, thus does not have Dynamics properties.